An amplifying circuit is made up of a number of preamplifying stages coupled to a final amplifying stage that drives, an output device. In an integrated circuit, these preamplifying stages often consist of a difference amplifier that employs NPN emitter-coupled transistors because of their superior frequency response. The output signal from the difference amplifier is taken off the collectors of the transistors and therefore has a higher common mode voltage than the input signal applied at the bases. With several preamplifying stages coupled directly together, the common mode output voltage at the output of the last stage is considerably higher than the common mode input voltage at the input of the first stage.
This increase in the common mode voltage must be remedied in most amplifying circuits before the final output signal is produced. For one reason, the power supply typically has fixed voltage limits or "rails" that cannot be exceeded for the circuit to work properly. Because an input signal voltage is added to the common mode voltage, the common mode voltage must maintain a sufficient margin within the rails to prevent distortion of the input signal. For another reason, it is often desirable that the input and output common mode voltages be the same voltage, typically zero volts. This voltage is usually the center of the power supply limits and makes separately designed portions of a circuit compatible without additional coupling circuitry.
To correct the problem of the increasing common mode voltage, voltage level shift circuitry is employed between stages. One of the simplest level shifting techniques in the prior art is the use of a capacitor to AC couple two amplifying stages together. The DC voltage is separately supplied in the second stage at a predetermined level to ensure that the common mode voltage is at desired level. AC coupling, however, cannot be used in circuits where the DC voltage forms an important part of the input signal because it eliminates that portion of the input signal. For example, in oscillography, it is desirable to display for analysis the slow variation of the DC portion of the input signal as well as the higher frequency portion.
A simple level shift circuit that includes DC coupling can be a Zener diode, coupled between the output of a first stage and the input of a second stage. A Zener diode, however, is extremely noisy, producing a random series of large noise spikes that degrade the integrity of the input signal. Compounding the noise problem is the location of the voltage level shifting in the preamplifying stages of a circuit. The noise generated by the Zener diode is amplified through all the stages. A Zener diode is also temperature dependent so that the amount of voltage drop across the diode changes as its temperature changes. These two effects render the gain nonlinear.
Another level shifting technique employs pn diodes in series, with a specific number chosen to obtain the desired voltage level shift. Series diodes do not generate the noise of the Zener diode, but they are highly temperature dependent, varying on the average of 2 mv/.degree. C. For example, if a five diode series were employed to shift the quiescent voltage of an amplifier 3.5 volts, the overall voltage shift could vary by up to 10 mv/.degree. C. A ten-degree change in temperature would cause the shift to vary 100 mv, or about 3% of the desired voltage shift. Diodes are also inductive, causing the impedance of the level shift circuitry to vary with frequency. The gain across the diodes is therefore nonlinear.
A third voltage level shift technique employs a resistor coupled between amplifying stages. A known current is drawn through the resistor to establish a predetermined voltage drop. The common mode output voltage of the first stage is applied at the high voltage end of the resistor, and the input voltage to the second stage is taken from the low voltage end. This technique, though simple in design, has major drawbacks. For one, a high-valued resistor limits the bandwidth of the level shifting and therefore introduces nonlinearities into the gain of the overall circuit at a low frequency. Decreasing the value of the resistor to broaden the bandwidth, however, requires that the current be increased proportionately to maintain the same voltage drop. But current in an integrated circuit cannot exceed a relatively low level or power consumption and dissipation become unacceptable. For another reason, the time constant of the parasitic capacitances in the integrated circuit and the input resistance of a current source must be balanced to maintain a linear gain by a time constant of the resistor and capacitance added in parallel. The input resistance of the current source is difficult to determine in an integrated circuit, which makes calculation of the required capacitance extremely difficult.
A fourth popular technique for level shifting employs a V.sub.be multiplier transistor circuit. By varying the resistances coupled between the collector and the base and the base and the emitter, the voltage drop across the level shifting transistor can be made to be any desired coefficient of V.sub.be within the limits of the power supply. This technique allows for greater accuracy in level shifting but suffers from the same temperature dependency and inductive effect as the series pn diodes circuit.
The inherent drawbacks of the prior art voltage level shift circuits such as these have limited the bandwidth of the amplifiers. The level shift circuit according to the present invention avoids these drawbacks and maintains linear gain for an amplifier across a broad bandwidth.